Wisconsin Sen. Ron Johnson on Wednesday was endorsed by the U.S. Chamber of Commerce, an important sign of support for the business-friendly lawmaker ahead of a difficult re-election test this November.

But it's unclear whether the behemoth business lobby -- and other well-funded Republican-aligned groups like it -- will actually spend big money on the GOP senator's behalf in a general election.

In a September interview with USA Today , leadership for the outside group American Crossroads said Johnson would need to run a “flawless” race to win re-election and wouldn’t commit to spending in the Wisconsin contest. The group hasn’t offered any more clarity since. On Wednesday, its spokesman declined to comment when asked whether it planned to air TV ads supporting Johnson’s campaign.

Johnson is widely considered the second most vulnerable Republican senator up for re-election this year, behind Illinois Sen. Mark S. Kirk. Johnson running in a state that traditionally votes Democratic in presidential years, and he's up against an opponent -- former Sen. Russ Feingold -- whose national profile as a progressive champion has helped him raise oodles of cash.

Early surveys of the race paint a grim picture for Johnson: Since April of last year, five of six polls from Marquette Law School have found Feingold sporting a double-digit lead, including a mid-February survey that found the Democrat winning by 12 points.

Johnson’s campaign has pushed back on those findings, arguing that the poll’s methodology is flawed. More importantly, they say that they’ve laid the groundwork for a campaign against Feingold -- who lost to Johnson in 2010 -- focused on national security and the former incumbent’s hypocrisy on campaign finance and other issues.

They’re hopeful those attacks will convince outside groups that spending in Wisconsin is a worthwhile venture, even as GOP incumbents fend off potentially majority-breaking challenges in states like New Hampshire, Ohio, Pennsylvania and Florida. The party is guaranteed to lose the Senate majority if it loses a net of five seats in 2016; the number shrinks to four if Democrats win the presidency.

There's hope, Johnson's supporters say, that the group plans to invest heavily on his behalf since he's the first senator in 2016 to receive a formal endorsement from the Chamber.

“Ron Johnson is a proven and tested leader who understands that the American Free Enterprise system is the only way to drive sustained economic growth,” said Rob Engstrom, national political director for the Chamber, in prepared remarks. “We’re honored to be in Wisconsin and stand with the state and local business community to highlight his record. Senator Feingold, on the other hand, is a career politician who has spent almost 30 years championing a ‘government knows best’ approach that stifles the American economy.”

But the Chamber doesn’t comment on strategic decisions like whether it will -- or won’t -- spend money on a Senate race.

The Wisconsin Republican could receive help elsewhere: The conservative favorite could find support from the Club for Growth, which has endorsed his re-election, or one of the many well-funded groups aligned with Charles and David Koch, such as Americans for Prosperity.